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中央構造線は活断層の分布からいくつかのセグメントに分かれると考えられている。これらの活断層を監視するために，四国の川之江，阿波池田，紀伊半島の粉河，五条の各地域で光波測量を実施した。20年間以上の中央構造線での光波測量の結果，各セグメントにより変動の違いが見られ，変動は中央構造線の活断層について述べられている右横ずれ変動の断層運動と調和している。しかし中央構造線での観測期間中の変動は，ひずみ速度で約1×E-7/年程度で，西南日本で平均して見られる弾性変形として考えられる範囲にある。, The Median Tectonic Line (M.T.L.) in west Japan is one of the greatest fault systems which consists of many active fault groups of the first class. For the purpose of monitoring of fault movement on M.T.L., we made 4 networks for Electro-optical Distance Measurement (E.D.M.) on M.T.L., that is, Kawanoe and Awaikeda on Shikoku Island and Kokawa and Gojo on Kii Peninsula The observations by E.D.M. at Kawanoe and Awaikeda networks have been carried out for over 20 years. The patterns of changes in each network differ from each other.Results observed in Kawanoe and Awaikeda show the displacement rate of 0-2.3mm/year and patterns of the changes correspond to right lateral fault movements that were inferred from research by trenches of faults and geological surveys. However, if we assume an uniform strain field in these networks, then the strain rate is about 0.1 micro Strain/year which corresponds to the average strain on normal fields in west Japan.

跡津川断層における光波測量は1971年に開始されて, ほぼ27年が経過した。その間に基礎石上への落石や耕地整理による地形の変化などがあったが、顕著な辺長変化はなく、定常的な断層変位を示唆するような系統的な変化は認められなかった。, The Atotsugawa fault is one of major active faults with right lateral displacements in central Japan. In order to detect the slip motion of the Atotsugawa fault, the Electro-optical Distance Measurements(E.D.M.) in central part of that fault has been carried out in the period 1971-1998.The results of the E.D.M. by the Geographical Survey Institute in central part of Atotsugawa fault in the period 1981-1997 show ceep motion of that fault(Tada, 1998). However, we couldn't detect significant slip motion of that fault during 27 years.

The observation system of ground strains, software for data analysis and the results in the period from 1984 to 1995 are summarized. At Kamitakara the ground strain has a high correlation with the change of groundwater level and the strain response to rainfall is controled by the groundwater level. It was found that the result from electro-optical distance measurements at Tokai-area shows a good correspondence with the ground strain at Kamitakara, and this fact suggests that the ground strains observed with electro-optical measurements might also have been disturbed by rainfall and groundwater.

In order to monitor the crustal movement in Kinki District, southwest Japan, we are continuing daily relative positionings with MINI-MAC2816 GPS receivers. The results show that a relative displacement of 5 cm or smaller will be successfully detected on the Uji-Shionomisaki baseline (162 km), which means that a linear strain of 3×10-7 is detectable. One of main errors in GPS observation is the excess path delay due to water vapor in the troposphere which is hard to be corrected to the level of a few cm by using only meteorological data on the ground surface. We introduced two water vapor radiometers and have started parallel observations with GPS receivers for developing effective correction methods for excess path delay due to water vapor.

The Median Tectonic Line (M.T.L.) in west Japan is one of the greatest fault systems which consists of many active fault groups of the first class. For the purpose of monitoring of fault movement on M.T.L., we made 4 networks for Electro-optical Distance Measurement (E.D.M.) on M.T.L., that is, Kawanoe and Awaikeda on Shikoku Island and Kokawa and Gojo on Kii Peninsula The observations by E.D.M. at Kawanoe and Awaikeda networks have been carried out for over 20 years. The patterns of changes in each network differ from each other.Results observed in Kawanoe and Awaikeda show the displacement rate of 0-2.3mm/year and patterns of the changes correspond to right lateral fault movements that were inferred from research by trenches of faults and geological surveys. However, if we assume an uniform strain field in these networks, then the strain rate is about 0.1 micro Strain/year which corresponds to the average strain on normal fields in west Japan.

The Atotsugawa fault is one of major active faults with right lateral displacements in central Japan. In order to detect the slip motion of the Atotsugawa fault, the Electro-optical Distance Measurements(E.D.M.) in central part of that fault has been carried out in the period 1971-1998.The results of the E.D.M. by the Geographical Survey Institute in central part of Atotsugawa fault in the period 1981-1997 show ceep motion of that fault(Tada, 1998). However, we couldn't detect significant slip motion of that fault during 27 years.

The observation system of ground strains, software for data analysis and the results in the period from 1984 to 1995 are summarized. At Kamitakara the ground strain has a high correlation with the change of groundwater level and the strain response to rainfall is controled by the groundwater level. It was found that the result from electro-optical distance measurements at Tokai-area shows a good correspondence with the ground strain at Kamitakara, and this fact suggests that the ground strains observed with electro-optical measurements might also have been disturbed by rainfall and groundwater.

In order to monitor the crustal movement in Kinki District, southwest Japan, we are continuing daily relative positionings with MINI-MAC2816 GPS receivers. The results show that a relative displacement of 5 cm or smaller will be successfully detected on the Uji-Shionomisaki baseline (162 km), which means that a linear strain of 3×10-7 is detectable. One of main errors in GPS observation is the excess path delay due to water vapor in the troposphere which is hard to be corrected to the level of a few cm by using only meteorological data on the ground surface. We introduced two water vapor radiometers and have started parallel observations with GPS receivers for developing effective correction methods for excess path delay due to water vapor.

From continuous observations of earth currents during a magnetic storm at two sites onthe Atotsugawa Fault, Nishiamo (NAM) and Miyagawa (MYG), and also from geomagneticvariations observed at the Kakioka station, the direction of the maximum change of electricself-potential has been determined by using the magneto-telluric (M.T) method.The maximum direction (M.D) was found to be E105N, which is nearly orthogonalto the strike of the fault, and the amplitude of the geoelectric variations at NAM was abouteight times as large as that at MYG.These observed results suggest that there exists a layer of high resistivity beneath theNAM station.From the geomagnetic and geoelectric observations the wave impedance, E/H has beendetermined as a function of frequency and the apparent resistivity in the vicinity of thefault has been estimated.

In order to monitor possible crustal movements directly related to the Atotsugawa fault, one of the first-class active geological faults in Japan, an observation with a biaxial boreholetiltmeter has started since 1978 in the Amo area located at the southwestern end of theAtotsugawa fault. The tiltmeter was set at a borehole with a depth of 3 m.A tilt step preceded by an anomalous gradual tilt was observed at the time of a localearthquake (M=3.6, ⊿=7 km). They might be interpreted as preseismic creep and coseismictilt step, respectively. However, a sudden rainfall just before the event might have causedthe apparent preseismic phenomenon. The observed tilt step does not agree with the tiltchange theoretically calculated from the fault model.Another tiltmeter of the same type has also been installed in the observation vault of theKamitakara Crustal Movement Observatory. The result shows that the instrumental drift ofthe tiltmeter itself is small enough to monitor crustal movements under favorable conditions, but its resolution limit is not sufficient for precise earth tidal observations.

The observation of total magnetic intensity was started with a proton magnetometer at Amo(NAM), near the south-western end of the Atotsugawa fault in the northern Gifu Prefecture, in June 1984. We analyzed the observed data by comparing them with the total intensity ob-served at Kakioka (KAK). Standard deviations of differences between two stations NAMand KAK are 2.6 and 0.7 nT for hourly and daily average values, respectively. Monthlyvalues fluctuate in a range of average 1 nT. The original records show a phase delay of 19minutes between the two stations, KAK and NAM. The telemetering system does not causeany drift during the observation period. Accordingly, it is concluded that local magneticchange of 2-3 nT will be detected by this observation system if such changes occur associat-ed with earthquake ocurrences.

The results of the electro-optical measurements of base-line lengths in two networks on theAtotsugawa fault, three on the Median Tectonic Line and one in the region near KamitakaraCrustal Movement Observatory are summarized. The accuracy of our survey is also discussed.If a change in some base-line length is detected over 10 mm+2 mm/km, then it may be con-sidered real with a reliability higher than 90 per cent. Two base-lines in the Atotsugawa net-work show a decrease of their lengths close to the above-mentioned limit of the observationalerror, which is consistent with the right-lateral displacement of the fault.

In order to detect possible variations in earth currents related to local earthquakes alongan active fault, continuous observations of earth currents have been in operation at two sites, Nishiamo and Miyagawa, (the distance between them is 17 km) on the Atotsugawa fault sinceAugust, 1982.In this brief report, we describe the installation of the observation sites including the lay-out of electrodes, design of electric circuits, together with some examples of the observed rec-ords which have been obtained to date. From these observations, relationsphips between dis-turbances of earth currents and of geomagnetic total forces have been investigated in somedetail.

The results from observations of ground strains and tilts, and from electro-optical measure-ments made at Kamitakara up to 1984 are reported in the present paper. It was found fromthose observations that both the average strain rates and tilt rates are smaller than 3x10 7/yearduring the last seven years.In order to increase the reliability of observations, crustal strains have been routinely ob-served with plural detectors attached to each quartz-tube strainmeter. The results of com-parison among them are also described.No significant precursory changes in the observed strains and tilts prior to the WesternNagano earthquake of Sept. 14, 1984, have been detected.

Since 1987, the total geomagnetic force and the earth's electric potential have beenobserved at the Horyu station in the pointed area of the Noto Peninsula, which projects intothe Japan Sea. The magnetic and electric data of the Horyu station were compared withthose of the other stations in the Hokuriku districts, and with those of the Kakioka station inthe Kanto districts. The following results were obtained.1. The simultaneous variation of the amplitude of total magnetic field at each station witha period between five and a few tens of minutes was recorded and compared. There existsa tendency for the amplitude of variation to increase as the observation site becomes furtherfrom the Japan Sea. Sasai (1969) analyzed the vertical component of geomagnetic fieldobserved at the several stations around the Kii Peninsula, which also projects into the PacificOcean. He suggested that the vertical component decreases according to the distance fromthe Pacific Ocean. The inverse trend of geomagnetic field in the regions close to the JapanSea and the Pacific Ocean was interpreted as the effect of induced magnetic field by sea water, introducing the schematic model of the "Island Effect".2. The maximum direction (MD) of the induced earth currents around the Horyu station wasobtained, using the data from the disturbance of geomagnetic field and/or lightning aroundthe station. A MD of N15' W was determined. In the case of active faulting as theAtotsugawa fault (Nakayama et al., 1987) and the Kurobishiyama fault, the MD vector wasobserved perpendicular to the azimuth of each fault. This was interpreted as the effect of thefracture zone of the fault having a low resistivity. The schematic model was introduced toexplain the MD for induced electric potential in the low resistivity area.

In order to investigate crustal deformations being concerned with earthquake activities in thenorthern part of the Chubu districts, Three observation stations, Horyu (Ishikawa Pref.), Tateyama (Toyama Pref.) and Suzaka (Nagano Pref.) were constructed. The main instrumentsset in the observation vaults are water-tube tiltmeters and strainmeters. Both of these instru-ments use differential transformers for their detectors.Twenty signals of crustal strains and tilts and some other geophysical data, and three signals ofseismic waves observed at Horyu are telemetered to the Kamitakara Observatory by an exclusivetelephone line with the signalling rate of 4800 bps.In the case of Tateyama or Suzaka, data measuring interval is ten minutes. Signals of crus-tal movements and other data are stored on RAM cassette in a digital data logger and collectedautomatically once a day at the Kamitakara Observatory by a public telephone line (1200 bps).All the data are stored in hard disk units and used for data processing

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In order to monitor possible crustal movements directly related to the Atotsugawa fault, one of the first-class active geological faults in Japan, an observation with a biaxial boreholetiltmeter has started since 1978 in the Amo area located at the southwestern end of theAtotsugawa fault. The tiltmeter was set at a borehole with a depth of 3 m.A tilt step preceded by an anomalous gradual tilt was observed at the time of a localearthquake (M=3.6, ⊿=7 km). They might be interpreted as preseismic creep and coseismictilt step, respectively. However, a sudden rainfall just before the event might have causedthe apparent preseismic phenomenon. The observed tilt step does not agree with the tiltchange theoretically calculated from the fault model.Another tiltmeter of the same type has also been installed in the observation vault of theKamitakara Crustal Movement Observatory. The result shows that the instrumental drift ofthe tiltmeter itself is small enough to monitor crustal movements under favorable conditions, but its resolution limit is not sufficient for precise earth tidal observations.

In order to detect possible variations in earth currents related to local earthquakes alongan active fault, continuous observations of earth currents have been in operation at two sites, Nishiamo and Miyagawa, (the distance between them is 17 km) on the Atotsugawa fault sinceAugust, 1982.In this brief report, we describe the installation of the observation sites including the lay-out of electrodes, design of electric circuits, together with some examples of the observed rec-ords which have been obtained to date. From these observations, relationsphips between dis-turbances of earth currents and of geomagnetic total forces have been investigated in somedetail.

The observation of total magnetic intensity was started with a proton magnetometer at Amo(NAM), near the south-western end of the Atotsugawa fault in the northern Gifu Prefecture, in June 1984. We analyzed the observed data by comparing them with the total intensity ob-served at Kakioka (KAK). Standard deviations of differences between two stations NAMand KAK are 2.6 and 0.7 nT for hourly and daily average values, respectively. Monthlyvalues fluctuate in a range of average 1 nT. The original records show a phase delay of 19minutes between the two stations, KAK and NAM. The telemetering system does not causeany drift during the observation period. Accordingly, it is concluded that local magneticchange of 2-3 nT will be detected by this observation system if such changes occur associat-ed with earthquake ocurrences.

Since 1987, the total geomagnetic force and the earth's electric potential have beenobserved at the Horyu station in the pointed area of the Noto Peninsula, which projects intothe Japan Sea. The magnetic and electric data of the Horyu station were compared withthose of the other stations in the Hokuriku districts, and with those of the Kakioka station inthe Kanto districts. The following results were obtained.1. The simultaneous variation of the amplitude of total magnetic field at each station witha period between five and a few tens of minutes was recorded and compared. There existsa tendency for the amplitude of variation to increase as the observation site becomes furtherfrom the Japan Sea. Sasai (1969) analyzed the vertical component of geomagnetic fieldobserved at the several stations around the Kii Peninsula, which also projects into the PacificOcean. He suggested that the vertical component decreases according to the distance fromthe Pacific Ocean. The inverse trend of geomagnetic field in the regions close to the JapanSea and the Pacific Ocean was interpreted as the effect of induced magnetic field by sea water, introducing the schematic model of the "Island Effect".2. The maximum direction (MD) of the induced earth currents around the Horyu station wasobtained, using the data from the disturbance of geomagnetic field and/or lightning aroundthe station. A MD of N15' W was determined. In the case of active faulting as theAtotsugawa fault (Nakayama et al., 1987) and the Kurobishiyama fault, the MD vector wasobserved perpendicular to the azimuth of each fault. This was interpreted as the effect of thefracture zone of the fault having a low resistivity. The schematic model was introduced toexplain the MD for induced electric potential in the low resistivity area.

In order to investigate crustal deformations being concerned with earthquake activities in thenorthern part of the Chubu districts, Three observation stations, Horyu (Ishikawa Pref.), Tateyama (Toyama Pref.) and Suzaka (Nagano Pref.) were constructed. The main instrumentsset in the observation vaults are water-tube tiltmeters and strainmeters. Both of these instru-ments use differential transformers for their detectors.Twenty signals of crustal strains and tilts and some other geophysical data, and three signals ofseismic waves observed at Horyu are telemetered to the Kamitakara Observatory by an exclusivetelephone line with the signalling rate of 4800 bps.In the case of Tateyama or Suzaka, data measuring interval is ten minutes. Signals of crus-tal movements and other data are stored on RAM cassette in a digital data logger and collectedautomatically once a day at the Kamitakara Observatory by a public telephone line (1200 bps).All the data are stored in hard disk units and used for data processing

The results of the electro-optical measurements of base-line lengths in two networks on theAtotsugawa fault, three on the Median Tectonic Line and one in the region near KamitakaraCrustal Movement Observatory are summarized. The accuracy of our survey is also discussed.If a change in some base-line length is detected over 10 mm+2 mm/km, then it may be con-sidered real with a reliability higher than 90 per cent. Two base-lines in the Atotsugawa net-work show a decrease of their lengths close to the above-mentioned limit of the observationalerror, which is consistent with the right-lateral displacement of the fault.

In order to detect the slip motion of the Atotsugawa fault, one of major active faults withright-lateral displacements in central Japan, six base-lines for electro-optical distance measure-ment were installed across the fault in and around Nishiurushiyama, Kamiokacho in 1971.The result from the re-surveyings in October 1972 and in November 1976 shows that for thefive years there have occurred no significant changes in all the six base-line lengths beyondthe observational error. This indicates that the fault in this part holds stuck or the sliprate, even if it slips, is less than 2mm/year. This is consistent with the extremely low seis-micity in that portion along the fault.